Mekong River Delta, Vietnam

Sixty percent of the delta is likely to become highly vulnerable to flooding if the sea level rises 17.7 inches (45 centimeters) above 1997 levels.

Increases in salinity from encroaching seawater may also reduce rice production, potentially reducing the opportunities for rice growing from three seasons in a year to just one.

By 2050, sea level rise in the delta could directly affect an estimated 1 million people or more.

Asia's many large river deltas are particularly vulnerable to sea level rise, which is accelerating around the world. During the twentieth century, global mean sea level rose at an average of 0.07 inches (1.8 millimeters) per year, but between 1993 and 2003 the average rate of sea level rise increased to 0.12 inches (3.1 millimeters) per year.

What the Future Holds

If we do nothing to reduce our carbon emissions, scientists project that the global average sea level could rise as much as 23 inches (59 centimeters) higher by the end of this century. If, on the other hand, we make significant efforts to reduce emissions, sea level rise between now and the end of the century could be limited to 15 inches (38 centimeters).

In Vietnam, the 1,544 square miles (40,000 square kilometers) comprising the Mekong River Delta—most of which lies barely more than 6.6 feet (2 meters) above sea level —could be flooded every year with a sea level rise of 3.3 feet (1 meter). Because around 78 percent of the delta's land is used for rice production, such flooding today would cause approximately $17 billion in economic losses—a substantial percentage of Vietnam's gross domestic product. By 2050, sea level rise in the delta could directly affect an estimated 1 million people or more.

The risk of flooding is likely to be especially severe during the wet season, which is marked by greater rainfall and downstream flow. The wet season lasts from May until November; September and October are already prone to pronounced flooding. By raising the level at which the Mekong meets the ocean, sea level rise could reduce the river's slope, decrease its flow, and increase flooding in the delta.

In one study, scientists looked at what might happen to the Mekong Delta as our climate changes. They based their projections on a sea level rise of 17.7 inches (45 centimeters) by the end of the century, assuming we do nothing to reduce carbon emissions. They found that, during the flood season, sea level rise would affect at least 69 percent of the delta by 2030, and virtually all of it by 2070. The scientists concluded that roughly 60 percent of the delta, including the whole coastal region and the Ca Mau peninsula, could be categorized as highly vulnerable to sea level rise.

Though rice is a water-grown crop, excessive water levels can literally drown the plant. Prolonged field inundation can lead to overly wet conditions during the harvest and could also cause delays in planting the next crop. A shift in the timing of planting or harvesting crops to avoid these conditions could mean a higher demand for water during the dry season, and may reduce crop yield.

Furthermore, increased salinity inhibits rice growth and can lower rice yield. With a higher sea level, salinity incursion would be more acute during the dry season, when the flow of fresh water is diminished, potentially forcing farmers in affected areas to switch to growing shrimp during those months. This could mean a reduction of the annual number of rice crops from three to one.

These effects of sea level rise can be exacerbated by localized sinking and settling of land (subsidence). Most deltas undergo subsidence, which can be caused by human activities such as well water extraction, dams, and diversions.

Among the dams that affect the Mekong Delta is the Pak Mun dam, built on one of the river's tributaries in Thailand. Dams like this can retain sediment that would otherwise replenish eroded or subsided land in the river delta. One-fourth of the Mekong Delta coastline is already undergoing erosion. More dams would likely increase this erosion as well as relative sea level rise, and might worsen water shortages and extend the area affected by salinity during the dry months.